Apparatus and method for monitoring intraocular and blood pressure by non-contact contour measurement

ABSTRACT

An apparatus and method is provided for monitoring internal pressure of a physiological vessel using non-contact contour measurement techniques. A pressure measuring system includes a light or acoustic source for impinging a beam on the vessel surface and a detector for detecting a reflected beam from the vessel surface, the system being mounted proximate to the vessel (e.g., an eye). The apparatus compares changes in vessel surface contour measurements with stored data relating changes in vessel surface contour with calibrated measurements corresponding to internal pressure of the vessel.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to methods and apparatus for non-contactmeasurement of internal pressure changes in physiological vessels orcavities.

BACKGROUND

Devices which measure blood pressure or the internal pressure of aphysiological vessel or cavity are well known in the art. For example,devices used for measuring blood pressure in clinics or offices aregenerally known as sphygmomanometers, while those measuring fluidpressure within an eye are generally known as tonometers. The latterinstruments measure the amount of tension on the eye's outer wall,allowing determination of fluid pressure within the eye. In order tomeasure outer wall tension, conventional tonometers often must havedirect or indirect physical access to the outer wall to deform, displaceor oscillate the outer wall. Analogously, measurement of blood pressurein most clinical situations requires application of an inflatable cuffto an arm or leg. In either case, special equipment is needed and,particularly in the case of tonometry measurements, the patient mustvisit a clinic or office for the measurement to be made.

Tonometry Principles

In general, most tonometers in use today work on either of twoprinciples. The first principle involves applying a known pressure orforce upon the wall and measuring the deformation produced. Instrumentsembodying this principle are known as impression or indentationtonometers. The second principle involves applying a known deformationupon the wall and measuring the force required to produce thedeformation. Instruments using the second principle are calledapplanation tonometers. Under either principle, the wall must bephysically manipulated, either by direct physical contact, such as witha probe or plunger, or by indirect contact, such as with an air puff oroscillating air stream.

Conventional tonometers are most often used for measuring intraocularpressure (IOP) by directly or indirectly flattening a pre-determinedarea of the cornea or the sclera. It is emphasized that though thenormal usage is for the pressure discussed here to be referred to asintraocular pressure that it is actually the differential pressurebetween the inside of the eye and the ambient pressure outside. Thisdifferential pressure is consistent with results from conventionaltonometry means. In order to contact these areas, the patient's eyesmust be closely aligned with the tonometer so that accurate readings canbe made. Such a procedure often involves a visit to a physician's officewhere a skilled operator performs the test. Furthermore, the operatormay have to anesthetize the eye causing injuries to go undetected whilethe eye is under anesthetic.

Need for Repeat Measurements

Compounding these difficulties is the requirement for repeatmeasurements of both IOP and blood pressure (BP) to clinically followthe course and treatment of disease. The clinical value of any singlemeasurement of IOP or BP may be reduced because long-term (e.g., weeksto months) pressure trends are always superimposed on shorter-termpressure variations. The latter variations can result, for example, fromchanges in body position, hydration or stress level. An additionalconfounding factor is the influence on IOP determinations of BPwaveforms within the eye. The presence of these factors may makeindividual pressure determinations by current clinical methodsrelatively unreliable. Peak pressures, which may be important inassessing the severity of a disease process or the efficacy oftreatment, may easily be missed because the measurement occurs before orafter the peak. Accordingly, unless fairly continuous tests areperformed over a relatively long period of time, measured BP and IOP maynot detect pressure changes which would be clinically important fordecisions related to the patient's health care.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by the apparatusand methods of the present invention. While conventional tonometers (aswell as sphygmomanometers) rely on invasive deformation of the vessel,the present invention provides long-term, repeatable measurement ofpressure in a physiological vessel without physically manipulating ordeforming the vessel by artificial means. In particular, the presentinvention determines internal pressure by non-invasive methods ofmeasuring the contour or geometry of the vessel and relating changes inthe contour to changes in pressure.

The present invention for non-contact determination of intraocularpressure and/or blood pressure trends is particularly convenient forhome or ambulatory monitor of glaucoma or heart patients. The principleof the invention is to use the eye itself as the transducer withnon-contact, non-intrusive means of detecting a "signature" proportionalto pressure that may be "learned" by calibration at measured valuesagainst a precision tonometry standard. Data for IOP is afforded bysignature analysis of contour changes at the limbus, related to IOP, bycomparison to calibration data stored in the patient's data unit or thephysician's office. Data for blood pressure comes from signatures ofreflections from blood vessels on the sclera or from the carotid arteryinternal to the eye, in comparison to a calibration data set taken inthe physician's office.

In preferred embodiments, an incident or measuring wave such as a lightwave or acoustic wave (including electromagnetic waves of differingmeasuring wavelengths) is directed to the surface contour of a vessel.Alterations in the light wave reflected from the surface indicatechanges in the surface contour, which in turn result from changes ininternal pressure of the vessel relative to ambient. Thus, the presentinvention can be utilized to measure internal pressure within anyphysiological vessel having an expandable or elastic wall, the geometryof which changes in response to changes in internal pressure.

According to one aspect of the present invention, changes in IOP(including changes in BP) may be determined by observing changes in thecontour geometry of the eye's outer surfaces in the limbus region (nearthe junction between sclera and cornea). Such determinations arepossible because as IOP fluctuates, a dense ring of fibers within thelimbus region (known as the annulus) tend to maintain the outerperimeter of the limbus. The annulus, while serving to anchor theciliary muscles (and thus the lens), also acts to stabilize the limbusregion and prevent the sclera and cornea from reacting to IOP changes asa single elastic membrane. Thus, while sclera and cornea expand andcontract relatively independently in response to IOP changes,corresponding measurable changes occur in the angle between scleral andcorneal surfaces. In preferred embodiments of the present invention,such angular changes are detected through their effect on the intensityand/or position of the beam or beams of light reflected from the eyewhen a light beam or beams are scanned across the scleral-corneal angle.Electrical signals proportional to changes in position of the reflectedbeam(s) may be provided, for example, by detectors comprisinglateral-effect photodiodes or charge coupled devices. Such detectors aresensitive to position change in a reflected beam due to a change inangle of reflection from the eye; detector outputs may then subsequentlybe related to the corresponding changes in IOP which caused the changein angle of reflection. Photodiodes and charge coupled devices canprovide electrical signals related to changes in reflected beamintensity and position, such signals as well as those fromlateral-effect photodiodes being adapted for direct input to digitalmemory, for transmission to a remote digital computer, or for additionallocal processing.

Those skilled in the art will appreciate that in addition to changes inIOP, changes in the internal pressure of any vessel with elastic wallsmay be determined by techniques analogous to those described above.Changes in wall geometry need only be related to calibrated measurementsof IOP and BP in the form of limbal contour signatures or pressureresponse contours respectively, values from which may then be used toestimate IOP and BP given only changes in wall geometry. For example,internal pressure changes in the carotid artery can be estimated byobserving through the lens of the eye the blood-pressure inducedconfiguration changes in the central retinal artery. Similarly, arterialblood pressure changes may also be estimated by simply observing, at asufficiently rapid sampling rate, similar angle changes to thescleral-corneal angle used to estimate IOP changes. In the latter case,blood-pressure waveforms may be electronically separated from otherpressure waveforms present within the eye (and detectible at thescleral-corneal angle) because of their relatively high frequency anddistinctive wave shape. Blood pressure also may be observed at thevessels on the surface of the sclera.

Short-term changes in the scleral-corneal angle (resulting fromcorresponding changes in IOP) can be quickly and easily measured andinterpreted with the apparatus and methods of the present invention,thus facilitating improved medical care. Signals generated by reflectedwaveforms striking the detector need only be compared with storedinformation in the form of signatures or contours. Values from the firsttype of stored information, called a limbus contour signature, allowconversion of alterations in reflected waveform electrical signals tochanges in IOP. Such stored information comprises experimentally derivedor predicted relationships between estimated IOP changes and alterationsin reflected waveform electrical signals (e.g., changes in reflectedbeam intensity or position). For each IOP application of the presentinvention (i.e., for each patient), calibrated measurements of IOPchanges may be stored and used to construct a unique limbus contoursignature relating alterations in reflected waveform electrical signalsto changes in IOP. Limbus contour signatures in most patients are stableover extended periods, thereby reducing the need for periodicrecalibration.

Analogous procedures are used to relate alterations in reflected beamintensity or position to changes in BP as determined bysphygmomanometer. Note that alterations in reflected waveform electricalsignals (due to intensity or position changes) may be accuratelyattributed to either IOP or BP using the unique characteristics of eachpressure waveform (e.g., frequency content and periodicity) to allowseparation and quantification. Obtaining numerical BP estimates byconversion of electrical signals attributed to BP changes isaccomplished by reference to a second type of stored information calleda pressure response contour. This response contour represents correlatedvalues of BP change and alterations of electrical signals representingchange in the BP component of IOP. Knowing the measure of alteration inthe electrical signal (whether due to change in intensity or position)allows one to estimate a corresponding change in BP.

Repeatable IOP and BP measurements are easily obtained in practice byplacing the apparatus of the present invention on a fixed plane or axisproximate to the eye; eyeglasses worn by the patient can provideconvenient mounting points. Limbus contour signatures and pressureresponse contours may be stored in a remote memory medium (i.e., in thephysician's office) to provide comparison with alterations in reflectedwaveform signals and thus to aid in the diagnosis and treatment of eyedisease. Pressure response contours may also be stored in a devicecarried by a patient, so that reflected waveform signals may be quicklyconverted to pressure measurements and the patient warned of anydangerous rise or trend in IOP or BP. In this application, for example,IOP rises not caused by normal physiological activity (i.e., heart beator body position changes) or the external environment (atmosphericpressure or temperature changes) can be detected by statistical movingaverages of angle measurements accumulated in the memory medium andinterpreted by reference to the limbus contour signature. Accordingly,the present invention is capable of recording changes in IOP or BPrelative to a baseline; trends in IOP and BP can be detected and warninggiven the patient (e.g., by audio alarm or vibrator) that immediatemedical treatment should be administered to prevent injury (e.g.,cardiac damage or certain complications of glaucoma).

Broadly speaking, the pressure measuring apparatus of the presentinvention comprises a light emitter placed proximate to a physiologicalvessel for emitting a light beam which impinges upon a portion of theouter surface of the vessel which may be anisotropic. A light detectoris spaced relative to the emitter for detecting alterations in areflected beam resulting from angular configuration changes in the outervessel surface, the light beam being reflected from a plurality ofpoints on the outer surface. The detector produces electrical signalsrelated to alterations in the reflected beam, and a signal processor maythen be coupled to the light detector for comparing reflected beamelectrical signal alterations with values from a limbus contoursignature or pressure response contour calibrated as a function ofmeasured pressure within the vessel.

The light emitter includes either a light emitting diode or a laser.Alternatively, acoustical or other forms of waves capable of reflectioncan be emitted rather than light. In either case, an appropriatetransducer converts relative alterations in the reflected wave intoelectrical signals which represent angular changes in the surface; thesignals may then be processed by the signal processor. Thus, inpreferred embodiments of the present invention, the scleral-cornealregion itself becomes a transducer for IOP and BP changes.

According to another aspect of the present invention, the light emitterand detector are coupled (as a transceiver) to a scanner which moves theemitter and detector in close proximity across the outer surface of theeye. The scanner includes a platform having the emitter and detectorfixed in spaced relation to one another, and a motive source or driveattached to the platform for moving it in close proximity across the eyesurface. Alternatively, the scanner may be stationary and the surface ofthe eye may move in relation to the scanner to provide the requisitescanning function. Such motion may be induced by normal eye or headmotion relative to an eyeglass frame on which the transceiver ismounted.

According to another aspect of the invention, the detector comprises atleast one photodetector configured to receive the reflected light beamand convert the beam to an electrical signal. At least one amplifier ofcommon circuit design is coupled to a photodetector for amplifying theelectrical signal. A local memory medium can be electrically coupled tothe output of the amplifier for accumulating the electrical signals,wherein the electrical signals correspond to changes in surfaceangularity represented by the light beam reflected from the outersurface of the vessel. Once accumulated in the local memory, theelectrical signals can be processed locally within the system toseparate and identify signals relating to changes in BP from thoserelating to IOP and downloaded for comparison locally or remotely withaccumulated sets of stored electrical signals (limbus contour signaturesand pressure response contours). In some embodiments, a remote computeris used for performing the necessary computations and for estimating IOPand BP as functions of, or relative to, alterations in a waveformreflected from the eye surface or from internal to the eye through thepupil lens. Those skilled in the art will recognize that estimates ofIOP and BP can also be made locally with a computer or processor carriedby the patient.

According to another embodiment of the present invention, an IOP/BPmeasuring apparatus is provided comprising a light emitter to be placedproximate an outer surface of an eye and at least one photodetectorspaced relative to the emitter. The light emitter preferably producesone or more light beams scanned across the outer surface at a limbusregion between or adjoining the sclera and cornea of the eye. Thephotodetector converts the intensity or position of light beamsreflected from the limbus region to corresponding electrical signalswhich are convertible by use of values from the limbus contour signatureand pressure response contours from the central retinal extension of thecarotid artery to IOP and BP estimates. Prior to conversion, the signalsmay be encoded to digital form by an analog-to-digital converter coupledto the photodetector. A local memory medium may be provided foraccumulating the digital data over a period of time commensurate withthe rate of changes in contour of the limbus region. Photodetectorsusable in the present invention comprise those sensitive to changes inintensity and/or position of an incident light beam, whether of visibleor non-visible light. Suitable photodetectors include, but are notlimited to photodiodes, lateral-effect photodiodes, and charge coupleddevices.

According to another aspect of the present invention, the light emitterand photodetector are coupled to a localized portion of an eyeglassframe movable in close proximity to the limbus region. The light emitterand photodetector are fixed in space relation to each other and moveablein relation to the limbus region. Reliance may then be placed on therepeatable involuntary movement of the eye in its socket in associationwith a turn of the head. Such eye movement will result in a scanning ofthe light beam from the emitter over the limbal region. Alternatively,one may employ prismatic transmission or faceted reflective deflectorsto, periodically or on command, deflect the light beam from the emitterto scan the limbus zone.

The present invention also contemplates a method for measuring IOP andBP which includes repeatedly scanning one or more light beams across alimbus region adjacent to and between the sclera and cornea of an eye,the surface of the sclera, or the central retinal artery. The light beamintensities and/or positions reflected from the limbus region, togetherwith separately determined (calibrated) IOP and BP determine the shapeof the limbus contour signature and pressure response contours and thusthe conversion from intensity/position data to pressure data. Periodicrecalibration of the signature and contour using independent pressuremeasurements gives assurance of accurate determinations of IOP; thespacing of such recalibrations depends on clinical estimates of theaccuracy of each conversion and periodic rechecks of calibration duringroutine office visits.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a cross-sectional view of an eye having a pressure measuringapparatus according to the present invention arranged in opticalcommunication with a limbus region of the eye;

FIG. 2 is a cross-sectional view of the limbus region of the eye havingcross-sectional contour geometries differing as a function ofintraocular pressure within the eye;

FIG. 3 is an embodiment of an optical reflective sensor according to thepresent invention arranged in close proximity with an eye's limbusregion;

FIG. 4 represents the output of the HBCS-1100 sensor applied as in thepresent invention to sense distance between the sensor and the limbusregion as a function of reflected photocurrent.

FIG. 5 is another embodiment of an optical reflective sensor accordingto the present invention arranged in close proximity with an eye'slimbus region.

FIG. 6 is a pressure measuring system according to the present inventionmounted in part on a patient's eyeglasses.

FIG. 7 is a processing flow chart illustrating conversion of contoursignature data into intraocular and blood pressure components forpatient monitor service.

FIG. 8 is a flow chart indicating the procedure for setting up,calibrating, and, in general, readying a system of type similar to aHewlett-Packard HBCS-1100, with extended focal length, for storinglimbus or blood vessel contour signatures in the field.

FIG. 9 is a flow chart typical of what might be set up in the data unitto tailor a system for a given patient.

FIG. 10 is a sequence of sketches illustrating key points in thegeneration of data by a single fixed-beam sensor system with electronicretina.

FIG. 11 is a diagram showing beam path over the electronic retina of asingle beam system as the beam crosses from sclera to cornea.

FIG. 12 is a sketch showing the signal generated from a single fixedbeam crossing from sclera to cornea as a function of rotation angle, ortime, during a typical data sequence.

FIG. 13 illustrates the generation of a vertical component of deflectionof a single beam reflection,onto the electronic retina by a specificangle of elevation.

FIG. 14 is a flow chart illustrating the setup, calibration, and fielddata acquisition of signals from discrete beam systems with electronicretina.

FIG. 15 views 1,2,3 are a series of views that illustrates the additionof sensors to extend coverage of a greater arc-length of limbus contourand/or increase the range of allowable elevation angles for field dataacquisition.

FIG. 16 is a block diagram illustrating a scheme for the processing ofdata, from either calibration or field acquisition, for diagnosis ofintraocular or blood pressure phenomena.

While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings are not intended tolimit the invention to the particular form disclosed, but on thecontrary, the intention is to cover all modifications, equivalents andalternatives falling within spirit and scope of the invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

There are two main physical principles in this invention; first, theangle of incidence of beams relative to a reflecting surface equals theangle of reflection; and second, that the structure of the eye is,geometrically, the intersection of two membranes of substantiallyspherical shape, following the laws of mechanics. These laws ofmechanics are comprised by a set of four conditions:

1. Stress-strain relationships (Hooke's Law)

2. Strain displacement relationships (Continuum of the eye)

3. Equilibrium conditions (fluid pressures/membrane stresses)

4. Boundary conditions (Ambient and physiological conditions)

The eye satisfies all of these conditions simply by its existence. It iscomprised of fluid filled dual membranes (Cornea and Sclera), of nearspherical shape, joined and reinforced at the limbus by a fibrous ringthat acts to react the forces of the ciliary muscles in changing theshape of the elastic lens to focus the image on the retina. The externalcontour of the eye is the result of the shape, size, and elasticity ofthe membranes and the differential pressure (IOP) between the inside ofthe eye relative to that outside (ambient) the eye. Similarly, thecarotid artery is a network of vessels of cylindrical form anchored tothe retina by elastic tissue; vessels on the surface of the sclera areof similar form.

The present invention affords determination of IOP, from limbus contoursignature; and/or blood pressure, from reflections off blood vesselsinside the eye, or on the sclera, and during the patient's daily routineto a degree never before possible. Detection of a high pressure eventallows medication to relieve the pressure before permanent damageoccurs.

The aqueous humor in the anterior chamber directly behind the cornealmembrane is a part of the eye's focusing apparatus and the source ofIOP. Aqueous humor is generated in the ciliary body to augment theoptical refraction of the lens; the change in thickness of aqueous inthe anterior chamber acts with the shape change of the lens to focus theimage on the retina. Aqueous is ported from the anterior chamber throughthe trabecular meshwork. IOP is transmitted to the vitreous humor byequilibrium of fluid pressure. The external contour of cornea and scleraat their junction, called the limbus, is determined by the differencebetween internal and ambient pressure (IOP); the stresses and strainswithin the corneal and scleral membranes; and the stresses and strainsin the fibrous reinforcement at the limbus. Eye structures are as uniqueas fingerprints between individuals, and the external shape of an eyemust follow the laws of mechanics, with the fluid pressure differencebetween the inside and outside of the eye in equilibrium with thestresses in the membranes and fibers. The geometric shape of an eye is afunction of IOP that can be learned from signature analysis ofindicators to that shape.

An eye is similar to a balloon, where size and shape depend on thedifference in internal and external pressure. The difference inpressures is reacted by change in tensile stress that stretches orallows the membranes to contract like the rubber in a balloon. For aneye to maintain its shape, the internal pressure must be greater thanthe ambient pressure; otherwise, the membranes would not be taut and theshape of the eye would be incoherent like that of an empty balloon.Ambient pressures vary widely for an eye; examples varying from that ofa fraction of an atmosphere for a climber atop Mount Everest, to that ofan extra atmosphere for every 30 feet of depth for a scuba diver whilediving. Though the absolute values of ambient pressure are different inthese examples, the differential pressure, IOP, is similar except forsecondary effects of the compressibility of the aqueous and vitreoushumors (fluids) themselves. For practical purposes these fluids areincompressible.

As previously discussed, the aqueous humor in the anterior chamber isthe source of IOP. Glaucoma is failure of the regulating system for IOP;usually associated with the inability to port the aqueous humor from theanterior chamber. The pressure generated by the aqueous humor istransmitted to the vitreous humor, inside the sclera, to equalize thepressure therein. The shape of the eye is the result of equilibriumbetween IOP, and the stresses in its physiological structure. The shape,then, is a unique function of IOP for each eye.

In summary, this invention is non-intrusive to the eye from anythingother than a beam of coherent energy, such as light, that will obey thelaw of optical reflection from the eye's surface. The physical featuregiving the greatest signal is the limbus, with maximum deflectionoccurring immediately on crossing the limbus' discontinuity. Theresponse of the eye to IOP is governed by the laws of mechanics that areindependent of the apparatus of the invention.

Measurement of Intraocular Pressure Changes

Referring now to FIG. 1, a pressure measuring apparatus 10 (partiallyshown) is brought in close proximity with a limbus region 12 of an eye14. Apparatus 10 is used for measuring contour displacements which isrelated to pressure within any physiological vessel having an elastic orflexible outer membrane which changes contour in relation to changes ininternal pressure. An eye 14 includes elastic membranes surroundingaqueous fluid 21 and vitreous fluid 22 which change internal pressureover a period of minutes, hours, days, months or years. Pressurereadings within eye 14 are preferably taken with reference to regionswhich change shape or contour in conjunction with changes in IOP.Specifically, limbus region 12 includes a fibrous ring structure regionnear the annulus 16 bound between cornea 18 and sclera 20. Changes inIOP of the aqueous fluid 21 and vitreous fluid 22 cause fluctuations inthe outer contour or shape of cornea 18 and sclera 20 respectively.

Pressure measuring apparatus 10 includes a light emitter 24 and lightdetector 26 fixed in space relationship to each other on a platform 28.It is to be appreciated that although light is the preferred reflectivemedium, other waveforms can be used to project reflected informationfrom limbus region 12. For example, sound waves or acoustical waves maybe used to provide analogous results, i.e., to present waveformalterations indicating relative positional changes in the outer contourof the limbus region 12 with the proviso that the beam displacementrelative to the eye be separately measured to construct the limbuscontour. Platform 28 can be arranged to scan in close proximity tolimbus region 12 in one direction or in both directions, as indicated inFIG. 1. Movement of platform 28 provides optical scanning of one or moreemitted and reflected light beams across the entire limbus region.Alternatively, platform 28 can be stationary and normal sweep movement(i.e., rotation) of the eye may provide the necessary scanning of thewaves across the limbus region 12. All that is necessary is thatapparatus 10 and limbus region 12 move or scan in relation to eachother, preferably along a single scanning axis, defined as the X-axis asshown in FIG. 2 and described below. The optical sensor provides asignal related to z axis displacement to the eye's surface to define thelimbus contour. Note that a plurality of detectors 10 (not shown) may bearranged to provide a holographic interferogram three dimensional realimage of the limbus region 12.

FIG. 2 illustrates the expansion or contraction of the outer walls inand around limbus region 12 as a function of pressure. For this example,the reinforcement at the limbus is idealized as being the dominantstiffness so that the annulus remains relatively inextensible, and thelimbus angle becomes more acute with increasing pressure. In actuality,due to the fact that eyes are unique physiological structures, this may,or may not, be the case. If the reinforcement at the annulus isrelatively soft in comparison to the corneal and scleral membranes, anincrease in pressure will tend to take the eye's structure in thedirection of becoming a sphere of uniform radius, or with the limbus'angle becoming less acute with increasing pressure. Specifically, risein aqueous pressure causes cornea 18 to increase from its outer position18a to 18b. Likewise, sclera 20 can expand from a low pressure position20a to a high pressure position 20b. Because of the constraint byannulus 16, the point or points of measurement of angles α₁ and α₂ arerelatively fixed on the eye, and it is one purpose of the presentinvention to measure angular changes α₁ and α₂ for the low pressurecontour and high pressure contour positions, respectively. Severalexamples of devices and methods by which the contour shapes can bemeasured during scan along the X-axis are described below.

There are various devices which can optically measure three-dimensionalcontour of an object. One form of optical contour sensor using lightemitting diodes and photodiodes may be purchased as Model No. HBCS-1100from Hewlett Packard, Inc. It is important to note, however, that anoptical contour sensor like the HBCS-1100 using light emitting diodes inconjunction with an aspheric lens as shown in FIG. 3 generally producespiece-wise linearity both before and after a set focal point distance.As will be described later, and illustrated in FIG. 4, linearity variesdepending upon the distance to the object relative to the point Zmax.Accordingly, while light emitting diodes used in Model No. HBCS-1100 areone form by which the present invention may be practiced, varying otherforms having desirable advantages may also fall within the scope andspirit of this invention.

Pressure measuring apparatus 10 illustrating Model No. HBCS-1100 opticalsensor 29 is shown in FIG. 3. Sensor 29 includes a transmission path 30formed between emitter 24 and detector 26. Sensing occurs by having anobject, in this case limbus region 12, placed at a distance along theZ-axis to obstruct transmission path 30, or complete path 30 byreflecting the emitter beam to the detector. In either case, thetransmissive or reflective sensing configuration allows non-intrusiveoptical readings be taken corresponding to the intensity and/or positionof the reflected beams.

The characteristics of the transmission path can be estimated throughthe use of an optical transfer function, OTF. The OTF is the ratio ofthe total optical flux transmitted to the amount of flux and theangularity (or position) of the beam reflected back to detector 26. Aswill be described below and illustrated in FIG. 4, the amount ofreflected optical flux or light received on detector 26 is optimum forthis embodiment when the nominal transmission path is set at a specificdistance.

As illustrated in FIG. 3, transmission path 30 represents a path oftravel between emitter 24 and detector 26. The path length is dependentupon the spacing between sensor 29 and limbus region 12 along theZ-axis. Placed along path 30 is a pair of lenses 32, an aperture 34 andglass window 36. At least part of apparatus 10 can therefore be packagedand sold as a single sensor unit 29, Model No. HBCS-1100, of which afull description is provided in Optoelectronics Designer's Catalog,Hewlett Packard (1985), pp. 1-39 to 1-44, the disclosure of which isincorporated herein by reference.

Apparatus 10, which includes an optical reflective sensor 29, determinesthe outer contour of region 12 by measuring the spacing betweenapparatus 10 and region 12 as a function of percent reflectedphotocurrent as illustrated in FIG. 4. Apparatus 10 can be designed suchthat an optimal spacing exists between sensor 29 and limbus 12 such that100% reflected photocurrent impinges upon detector 26 at a particularspacing distance (Z_(max)) . As apparatus 10 moves relative to region 12along the X-axis, the percent reflected photocurrent will eitherincrease or decrease depending upon whether the transmission path isadvancing toward or away from, respectively, the optimal path length.Z_(max) is preferably set at a relatively fixed Z-axis distance betweensensor 29 and the annulus region 16. On either side of Z_(max), percentreflected photocurrent decreases from the optimal 100% as shown in FIG.4. Zones 39 and 41 are piecewise substantially linear segments thatprecede and follow the maximum photocurrent point at the focal point ofthe aspheric lens. This means that the sensor, in order to avoidduplicate outputs over its range of operation should be positioned tofunction solely in either zone 39 or zone 41 so that the output sloperemains monotonic over the range of operation. Zone 41 has advantagesover zone 39 for the present invention because its range is greater thanthat of zone 39 (both absolute and usable distance from the limbus isgreater) and the sensor would have greater clearance with the eye orlashes. Zone 39 has an advantage over zone 41 in that it is of greatersensitivity and linearity, with a positive slope. The limbus contoursignature (and pressure response contour) will allow accurate estimationof IOP and BP in spite of the non-linearity regardless of the zonechosen. The repeatability of the zone used, however, is important tothis application. Lens characteristics may be modified to tailor thepreferred zone for use.

The sensor reflector distance or transmission path length used inHewlett Packard Model No. HBCS-1100 is fairly short and narrow. However,a longer or a broader range of detectable distances, Z, can be measuredembodying the principles of Model No. HBCS-1100. For example, emitter 24output can be amplified and different lenses 32 can be used to refocusthe beam so that the sensor 29 can be placed from 2 millimeters toseveral centimeters away from the vessel or region 12. Other forms ofphotodetectors can also be used. The most popular types ofphotodetectors suitable for use with the present invention include:Charge coupled devices (CCD's), PIN photodiodes, lateral-effectphotodiodes or avalanche photodiodes. Detector 26, using a highlysensitive avalanche photodiode of common design, provides internal gainto the resulting electrical signal thereby useable for detectingreflected waves when path lengths are relatively long. Photodiodesprovide optical-to-electrical conversion resulting in an analog currentwhich can be manipulated using conventional circuit techniques. Inparticular, electrical signals from the photodetectors can be convertedfrom analog-to-digital (A/D) format using standard converters such as asuccessive approximation A/D converter or a high speed A/D flashconverter.

Example 1

A general embodiment of the present invention is illustrated in thefollowing example. This example uses the sweep of a beam over the limbusat either approximate right angle, or tangency, to the limbus to producethe limbus angle signatures required. In the initial case of sweep atnear right angle to the limbus, the eye may be stationary while the beamsweeps relative to the eye. In the case where the beam sweeps tangent tothe limbus, it is necessary to rotate the eye about a vertical axis sothat the limbus crosses the plane of sweep. This is an embodiment thatwill accomplish the same effect as the subsequent multi-beam unit ofEXAMPLE 4, through the kinematic inversion of sweeping a single beamthrough an essentially similar multiplicity of position, in lieu of amultiplicity of beams in fixed positions.

Laser Measurement of Eye Contour

Utilization of a laser for three-dimensional contour measurement isillustrated in FIG. 5. In particular, a three-dimensional opticalmeasuring technique can be employed as described in U.S. Pat. No.4,935,635 (herein incorporated by reference). Three-dimensional contourmeasurement includes a laser diode 42, polygonal reflector 44 andphotodiode array 46. Further included is a linear stepper motor 48having two shafts, one shaft for providing rotation to reflector 44 andthe other shaft for driving a threaded screw cam attached to moveableplatform 28. The laser 42 and photodiode array 46 functions similar tosensor 29 of FIG. 3 in that relative spacing along the Z-axis betweenapparatus 10 and limbus region 12 are sensed to provide atwo-dimensional contour reading. The position of the returning imagedbeam spot along the length of photodiode array 46 indicates the contouror Z-axis distance between the particular point on region 12 andapparatus 10.

Each measurement of intraocular pressure is achieved by performing onescan of platform 28 across limbus region 12. Each scan produces areflective beam positional change upon array 46 as the beam travelsacross limbus region 12. As the contour changes during each scan, theangle of incidence changes and the corresponding reflected wave positionupon the array changes. It is the relative change in the position uponthe array 46 that determines a proportional difference in depth sensedon the eye surface. This technique of depth detection to measurethree-dimensional contour is commonly described in Patent No. '635 as"triangulation". The y-axis dimension is afforded by separate sweeps atincremental changes in y.

An encoder such as, for example, an analog-to-digital converter 50counts each photodiode on a pixel-by-pixel basis as it is scanned fromthe photodiode array 46. The resulting counter value representation ofdigital data is latched and stored into a local memory medium 51,whereby it can be later read by a signal processor 52, illustrated inFIG. 6 and described below.

The choice of local or remote processing of reflected waveform signalsmay depend on availability of adequate computing power near where themeasurements are generated. In this regard, neural or "neuron" networkelectronic chips which are now available may influence the choice. Onesuch device contains three microprocessors, several channels ofinput/output (I/O) communications and significant on-board random-accessand read-only memory.

Neural chips, in combination with proper on-board software, are thuscapable of converting reflected waveform signals to estimated IOP and BPchanges by application of values derived from the limbus contoursignature and/or the pressure response contour. Processing of anglechange (reflected waveform) data to yield estimated IOP and BP inconventional units of measurement through application of a pressureresponse profile is also possible on the chips. Thus, for example,either physician or patient may obtain an IOP or BP readout in mm-Hg innearly real time. The various I/O options make it possible to provideappropriate warnings to the patient and even to calculate proper dosageof medication and administer it automatically. Simultaneously, suchchips may process data for storage in an on-board memory or for directtransmission to a physician's office via radio telemetry or modem andland line. Such transmission would allow prompt interpretation of thedata by skilled medical personnel; impending acute exacerbations ofglaucoma or arterial hypertension may be monitored closely and treatedpromptly to avoid or reduce morbidity.

General operation and setpoints for the counter of analog-to-digitalconverter 50 and latches within medium 51 are determined based uponwhich pixel on the array is currently being interrogated. Other countersmay also be available to determine X-axis position of platform 26 viastepper motor 48 and X-axis position of platform 26 in conjunction withpolygonal reflector 44 position. Thus, latched digital datacorresponding to electrical signals placed in memory 51 also provideindicia of the relative position of the X and Y scanning axes viaconnection to motor 48.

The light transmitted from laser 42 has a coherent signature which issufficiently unique to distinguish it from ambient light. The angularcontour signature of limbus region 12 is indicated every time the eyerotates about its vertical axis far enough for the limbus to passthrough the beam. An inclination of the eye about an axis in thehorizontal plane results in deflection of the beam in a plane normal tothe normal scan plane. This data may be recorded to allow calculation ofthe eye position as well as the limbus signature for subsequentdetermination of intraocular pressure.

FIG. 6 illustrates a pressure measuring system 60 which includes anapparatus 10 mounted proximate to a patient. System 60 also includes aremote processor 52 capable of being coupled to apparatus 10. Apparatus10 is preferably mounted within or proximate to the vessel region.Specifically, apparatus 10 can be wholly or partially mounted within oronto, e.g., the frame of a pair of eyeglasses 62 placeable upon apatient undergoing intraocular pressure measurements. Platform 28 can besecured in moveable relation to a corner 64 of the eyeglass frame.Platform 28, containing emitter 24 and detector 26 is moveable betweeneyeglass lens and eye 14 in close proximity to and over limbus region12. If a light-emitting diode similar to that used in Hewlett PackardModel No. HBCS-1100 is used, the entire packaged sensor can be mountedon platform 28 and directed toward limbus region 12 between eye 14 andeyeglass lens 66. Alternatively, if a laser is used, similar to thatshown in FIG. 5, reflector 44 and array 46 can be mounted upon platform28 having a motive source provided via cable 68 coupled to motor 48.Laser diode 42 is preferably placed within a package 70 which housesmotor 48, laser diode 42 and a local memory medium 51. A battery (notshown) may be included within package 70 to supply power for operationof apparatus 10.

Cable 68 therefore can provide a rotatable mechanical cable for drivingplatform 28 as well as an optical wave guide for transmitting laserenergy from laser 42. Alternatively, if the optical emitter and detectorare fully contained upon platform 28, as shown in FIG. 8, the electricalsignals transmitted to the emitter and from the detector are containedwithin an electrical conductor within cable 68. Thus, depending upon theconfiguration desired, i.e., whether a laser or LED is used or whetherthe laser is mounted on platform 28 or on package 70, cable 68 mayinclude an electrical conductor, fiber optic cable, or both. Cable 68also preferably includes a rotatable cable which transmits mechanicalrotation from motor 48 to translational movement of platform 28 androtational movement of reflector 44.

Eyeglasses 62 can be of common design generally adapted to fit in fairlyclose proximity to the outer surface or contour of eye 14. Eyeglasses62, being fairly stationary in relation to eye 14, provides a relativelystable and repeatable positioning tool by which long term and continuouscontour measurements can be taken. Eyeglasses 62 can be worn over aperiod of days, months or even years thereby allowing access for longterm intraocular pressure measurements. The operating distance betweenthe platform movably fixed to eyeglasses 62 and eye 14 can varydepending upon various hardware chosen. However, the present designallows contour measurements at varying operating distances anywhere fromseveral millimeters to several centimeters, or even far beyond as in thecase of nonphysiological applications.

During each measurement routine, platform 28 can be activated to scan inthe X-axis across eye 14 and, in particular, across limbus region 12.Alternatively, it is within the scope of the present invention thatscanning can be equally achieved by maintaining platform 28 in a fixedposition and naturally moving the eye's focal point along the X-axis. Ifplatform 28 is movable to provide the scanning function, eye 12 mustremain fixed in relation to the moveable platform. Thus, the eye can befocused at a fixed point during each scan routine so that repeatablemeasurements can be taken. A focus point can be provided by attaching atarget to eyeglasses 62, whereby the patient maintains fixed eyeconcentration upon the target during each scan routine. Consequently,each scan presents a scan slice within the X- and Y-axis. Furthermore,providing eyeglasses 62 do not slide a substantial distance down thepatient's nose, fixed position along the Z-axis is also maintainedbetween measurement scans.

A first set of values representing the limbus contour signature relatingalterations in the reflected waveform angle or intensity(electromagnetic or acoustic) to IOP is stored in a first remote memorymedium 54 such as a floppy disk, compact disk, etc. A second set ofvalues representing the calibrated pressure response contour relatingalterations in reflected beam intensity or angle to changes in BP isstored in a second remote memory medium 56, similar to medium 54. IOPmeasurements used in performing the calibration are obtained with aconventional tonometer applied approximately simultaneously with anoptical scan of the limbus contour. BP measurements are analogouslyobtained with a conventional sphygmomanometer. The data obtained duringthe optical scan corresponding to IOP and BP readings are then stored ascalibration data within media 54 and 56. A physician may induce severalpressure changes within a patient's IOP or BP to establish a broad rangeof calibration points.

Signal processor 52 is placed in a remote location from the patient,preferably in a physician's office. Processor 52 includes a computerwhich can receive downloaded data from local memory medium 51 andcompare that data with data stored in remote memory media 54 and 56. Thepatient can download data from medium 51 through a modem connecting thepatient's residence to the physician's office. Alternatively, thepatient may visit the physician's office and physically connect outputvia an RS232, IEEE488, or other port from medium 51 to processor 52.Processor 52 may be a personal computer having external computer businput and read/write data capability.

Digital representations of reflected light beam intensities andpositions are convertible to changes in IOP and BP through applicationof the limbus contour signature and pressure response contour. Anexample of the conversion process is represented by the processing flowchart shown in FIG. 7. During each scan, IOP measurement data (as storedin medium 51) are entered via input lines 79 to processors 80 and 83. Inprocessor 80, BP waveforms are separated by characteristic wave shapeand/or frequency content to be sent on to comparator 82. Thereafter, IOPwaveforms routed to comparator 83 are compared therein with the storeddigital representation of the limbus contour signature (as stored inmedium 54), while BP waveforms input to comparator 82 are comparedtherein with the pressure response contour (as stored in medium 56) toobtain IOP and BP respectively. Comparison in each case may comprise atable look-up with interpolation of previously correlated IOP or BP dataas stored in medium 51 with separate calibrated measurements of IOP andBP respectively, said measurements being made at substantially the sametime as the correlated IOP and BP data are taken at the detector (e.g.,photodiode array 46). Outputs of comparators 82 and 83 representingestimated BP and IOP respectively are processed for display, warning,storage or subsequent digital processing in processor 84. Thus, aspressure fluctuates throughout a day, week or year, measurements can bechecked to determine if pressure calculated from each contourmeasurement exceeds a pre-determined amount. If so, the patient isimmediately apprised of the situation so that he or she can administermedication and/or seek medical treatment. Moreover, processor 52 canprovide direct dosimetry information for medications needed to achievemore acceptable pressure readings. By monitoring rapid fluctuations inIOP or BP or long-term trends in pressure, the present inventionprovides a more convenient and accurate monitoring of pressure so thatmedication is more effectively dispensed. Timely intervention can thenprevent or delay important complications such as blindness (fromincreased IOP) or stroke (from increased BP).

A functionally identical scheme was used to evaluate this concept by theuse of a hand-held, Symbol ™, laser bar-code scanner. The output of thescanner was used to trigger an oscilloscope simultaneously with its beamsweep. The reflections were recorded on a video-recorder approximatelyaligned with the axis of the reflected beam. Though the sweep of thecamera could not be synchronized with the beam sweep, the pulse at thebeam crossing the limbus could be observed, on the oscilloscope from theoutput of the camera's video jack, by manually adjusting the sweepvernier to catch the limbus crossing within the camera's rasterizedimage. In this manner, the output change at the limbus could be measuredwithin approximately one (1) mm of trace deflection, or to 0.1 V.

A recently slaughtered pig's eye that was pressurized from approximately4 to 31 mm-Hg by needle and syringe as measured by a Schieotz tonometerwas scanned. The pulse generated by the laser beam crossing the limbuswas a sharp peak of less than a millisecond duration, but of repeatablecharacter and amplitude. The output pulse heights on limbus crossingvaried from 0.5 V at 4 mm Hg, to 1.0 V at 14 mm Hg, and 2.4 V at 31 mmHg.

Example 2

The following example illustrates the apparatus and method employed inmeasuring changes in contour and relating such changes to pressurewithin the eye. Any method of measuring limbus contour of sufficientresolution to define IOP is suitable. This example is provided frominitial efforts to identify existing sensors to verify the concept. Onesuch device is the Hewlett-Packard HBCS 1100, a photoelectric sensorwith an integral light source of specific wave length, or color, and adetector that measures the light reflected from the target through anintegral lens, designed to optically couple emitter and detector. Thissensor is used to read digital bar codes, measure thickness of sheetmaterials, or detect the presence of a sheet in a feed mechanism, etc.

Preliminary Measurements on Eye Models and Human Eye

This apparatus was tested initially using a Hewlett-Packard HBCS-1100sensor. It was used with precision sweeps past the "limbus" of anacrylic model of a human eye; and with manual sweeps of the beam pastthe limbus of an actual human eye. The data from the output of thesensor in sweeping the model eye were recorded on one axis, with theoutput of a sweep position potentiometer on the other axis, of an X-Yrecorder for three successive sweeps with slight repositioning of theinitial point between the tests. The results were three separate traces,displaced slightly, that tracked each other with nearly perfectlyparallel separation. There was a reversal, or notch, at the instant ofcrossing the limbus that was identically repeatable. Under microscopicevaluation it was determined that there was a scratch in the plastic atthe limbus that gave the notch. This scratch was visible only undermagnification. Subsequent inspection on an optical comparator, at 40×magnification, indicated that the scratch was of less than 1/10,000(0.0001) inch in depth, yet it produced a trace deflection on the x-yrecorder output of over 1/2 inch. The low intensity LED beam also waspassed over the limbus of the human eye and the output was qualitativelyobserved on an oscilloscope with large scale deflection. The output wasreproducible at the limbus crossing.

The HBCS-1100 is not ideal for direct application due to its short focaldistance of less than 0.1 inch that would require that it be mounted tooclose to the eye for practical use. However, the aspheric lens may bemodified to longer focus (Zmax) distance. While this is generally anexpensive and time-consuming process, a similar sensor, with integraland sealed emitter/detector as a single unit, is satisfactory for thedescribed application. This unit is necessarily fixed in its opticalrelationship and is, therefore, adaptable to change in prescription forpatient's need only by Z axis placement of the unit or angular alignmentof the optical axis.

FIG. 8 is a flow diagram that illustrates a setup, calibration, and dataacquisition scheme for a sensor of the same type as the HBCS-1100, butwith a longer focal length lens. The integrated assembly of this devicefacilitates the installation and adjustment of the unit, but limitsflexibility. It may be desirable to employ a separate sensor for themeasurement of x-axis displacement as shown in FIG. 3, since the lenssomewhat masks the effect of the discontinuity at the limbus withoptical interference. This probably could be overcome with additionalstudy of lens characteristics. As shown in the flow diagram of FIG. 8,the first task is to set the HBCS type sensor in position relative tothe eye for producing the best possible signature from limbus crossing.The x-axis transducer, which may be a linear potentiometer, or a DCDT(direct current displacement transducer, a differential transformer withsolid state oscillator and demodulator, etc.) is also set up to definethat component of the contour profile. The data unit will include therequired signal conditioning for both sensors along with the powersupply for all elements in addition to that shown in the flow diagramFIG. 9 for the patient's pocket data acquisition unit (hereinafter the"data unit") function. The flow charts of FIGS. 8 and 9 show steps thatmay be taken to calibrate such a system for IOP measurement and ready itfor data acquisition in the field. The parts for accomplishing the itemsin both of these figures are of common usage in the field and may beaccomplished by numerous combinations of components by one skilled inthe art.

Example 3

In order to obtain a system that may be specifically tailored to apatient's specific requirements, a system with greater flexibility isrequired. Since IOP measurement is a function of reflections fromessentially discrete spherical surfaces, beams incident the sclera nearthe limbus will give discrete reflections as the eye rotates so that thelimbus crosses the fixed beams. These beams may be positioned inplacement and angle to produce desired incidence. Additional flexibilityfor prescription is afforded by a separate photodetector that may bepositioned independently of the emitter source.

Description of this embodiment is simplified by considering a singlebeam source, initially. This is not overly simplistic since such asystem is capable of making useful measurements, in the physician'soffice as well as in patient use and, in fact, is expected to be thepreferred embodiment for most patients. In this embodiment, ascalibration signals are recorded at several values of IOP, they also arerecorded at several angles of elevation of the line of sight for eachIOP.

Single Beam Example

FIG. 10 shows a system with a single beam in fixed relation to the eye'ssocket (center.) The system is configured by the physician'sprescription to meet the patient's needs; and this figure shows atypical setup.

The lateral axis of the eye in FIG. 10 is horizontal and in the plane ofthe paper. The view looking down this lateral axis is not shown since itis illustrated in the three view sketches of subsequent discussion ingreater detail. The beam path, and its reflection, lie in the horizontalplane of the eye's symmetry so that both incident and reflected beamsshown in this figure are in the plane of the paper. For this idealizedexample, this array would give a straight, horizontal trace over thesurface of an electronic sensor that, in effect, is the "retina" of thedevice (hereinafter referred to as ER for electronic retina).

View 1 of FIG. 10 shows the eye looking straight ahead with the beamadjusted to reflect off the sclera and onto the ER at point "a". Theangles labeled φ1 represent the angles of incidence and reflectionrelative to the surface of the sclera. To acquire data for eithercalibration or data acquisition consistent with this scheme ofcomponents, the eye is rotated substantially about the vertical axis ofthe sclera. This rotation may during routine motion of the eye bedirected by an image moving in plane, in front of the eye, or by amaneuver performed by the patient in rotating his head about a verticalaxis in opposite sense to the desired eye rotation. The resultinginvoluntary rotation of the eye is a motion that is easy to reproduce byboth sighted and blind patients. Since the sclera is nominallyspherical, and the eye rotates about the center of the sclera, there islittle deflection of the beam prior to its contact with the limbusexcept from the surface roughness of the sclera.

In View 2 of FIG. 10 the beam has just crossed the limbus onto thecornea with the result that the beam angles of incidence and reflectionare now at φ2. The extreme deflection of the beam relative to the ER, topoint "b," occurs here. This maximum deflection of the beam, from a tob, is the analogy for the measurement of IOP; the axis definingincidence and reflection has shifted from sclera to cornea.

View 3, FIG. 10 illustrates that beam deflection to "c," as the eyecontinues to rotate clockwise is such that φ, now φ3, is diminishing.The increased rate of beam deflection per unit of eye rotation is due tothe shorter radius of the cornea (relative to that of the sclera), thatgoverns beam reflection at this orientation.

In the real case, the limbus is not purely angular, and the actual stepfrom a to b is "softened" by the slightly radiused contour of thelimbus. The beam path, recorded by the output from the ER, may bereduced to IOP units; either directly, from a stored calibration tableor function in the patient's data unit; or indirectly, where the data isstored on portable medium in the data unit for subsequent comparison tocalibration data in the physician's office. Medication is authorizedaccording to prerecorded instructions in the data unit, or by telephone,modem, pager, etc.

FIG. 11 shows the ER with the points labeled as described above.

FIG. 12 shows typical output from the sensor, giving beam displacementrelative to rotation of the eye; The deflection from a to b is relatedto IOP by the calibration data. The deflection of the beam in thevertical plane is a function of the elevation of the angle of sight,making each image step at the limbus and its angularity uniquelyrepresentative of both limbus contour and elevation of line of sight.

FIG. 13 shows the effect of approximately 5° of elevation of the angleof sight on the data. The corneal outline shown in dashed line in thefront view gives horizontal line of sight; all in solid line iselevated. The normal to the cornea (its radius since it is spherical)establishes incidence, and the result is that the beam is reflected froma to b'. The angle between a to b' and the axis of the ER (a to b) isproportional to elevation angle. The signature on the ER, then, consistsof the step from a to b' that is proportional to IOP; and the angle of ato b', proportional to elevation. The "softening" of actual limbuscontour diminishes the slope derivative at the limbus and provides apath over the ER of discrete path and longer duration, making trackingeasier than if it were a true discontinuity. Note that if the cornea andsclera are truly spherical there is no need to measure elevation forseparate entry for calibration.

The only difference between calibration data and that for actual patientmonitor is that, for calibration, data are taken in the physician'soffice at several levels of IOP induced, by medication, and with"conventional" tonometry used to measure actual IOP's against which beamdeflection is compared. This gives calibration curves of IOPs asfunctions of peak beam deflection and angularity. FIG. 14 is a flowchart that illustrates a setup, calibration, and data acquisition schemefor systems with discrete beam sources and ERs that use the deflectionat the limbus discontinuity as the primary transduction principle. FIG.14 is valid for both single beam and multiple beam systems with the onlydifference being the number of sources aimed at the eye's surface.

This single beam system was tested by using an Apollo MP-1600 laserpointer through a pin-hole aperture for beam sharpening; again, with apig's eye for analysis. The pressure was from a reservoir connected tothe eye by IV connection, and the pressure equivalent in mm-Hg was setrelative to the pupil, with water column height. The optical arm fromthe eye to a sheet of graph vellum that served as ER was approximately3.5 in. In taking the data, the beam was swept past the limbus by avernier on a precision height gage. The angle of incidence was estimatedat 10-15° off normal to the sclera. The resulting data are shown below:

    ______________________________________                                        SINGLE BEAM DATA2                                                             IOP P-mm-Hg  Trace Deflections - mm                                           ______________________________________                                        0            36                                                               10           63                                                               20           95                                                               30           115                                                              ______________________________________                                    

Example 4

The single beam method will handle the majority of patients' needs.Multi, or swept beam devices are feasible also. One reason for themulti-beam embodiment is in the case where there is substantialvariation of limbus contour over the small arc where the contoursignature is to be taken and the definition from a single source is notdiscrete; Another is an increase in the range of the angle of elevationof the line of sight for automatically acquired data. These points alsoare true for the inversion case where a single beam is swept as in abar-code scanner or Example 2 so that it touches or crosses the limbuswith the eye stationary.

Multi-Beam Systems

FIG. 15 shows a three-beam configuration that produces reasonablesignatures over a broad range of elevations of line of sight. Theprinciple, illustrated in FIG. 15 shows progression through three setsof sketches as the eye rotates so that the limbus crosses the beamarray; the array being fixed in relation to the eye socket. The top viewof View 1, FIG. 15, shows that the beams for this example are aligned atan angle of incidence relative to the radius of the sclera so that thereflections fall on the ER as shown. It is emphasized that specificgeometry and number of beams will be by prescription of the physician.In this example, three beams are directed normal to the surface of thesclera as seen in the view looking directly into the cornea, with theresult that the reflections are at points a1, a2, and a3 on the ER.These designations signify:

1. The letters a, b, or c mean that the reflections are from sclera, thecorneal edge of the limbus, or the cornea surface, respectively.

2. The numbers 1, 2, and 3 indicate the specific beam causingreflections shown in the sketches.

View 1 is the initial setup and the reflection positions remainessentially constant, except for deflections over blood vessels or otherroughness of the sclera, as the beams move over the substantiallyspherical sclera when the eye rotates in its socket. As the limbuscontacts beam 2, as shown in View 2, the point of reflection on the ERjumps from a2, to b2, as the spherical surface governing reflectionshifts from sclera to cornea at the limbus. The top view of View 2illustrates how the angles of incidence/reflection shift as beam 2reaches the corneal side of the limbus. Due to the circularity of thelimbus in the front view and the linear array of beams incident thesclera, beams 1 and 3 are undeflected. The deflection from a2 to b2 isthe primary analog for measurement of IOP.

As rotation of the eye continues further in the same direction, as inView 3, beams 1 and 3 contact the limbus at a1 and a3, and are reflectedto b1 and b3 as shown in View 3. In this example they are reflectedbeyond the surface of the ER, however, the path angles are preserved,confirming angles of reflections that result from off-center contactwith the limbus. Their symmetry confirms normal contact between beam 2and the limbus; hence "zero" elevation angle. Deflections from a1 to b1or a3 to b3 are, or would be, separate and corroborating measures ofpressure to that of beam 2. (Beam 2 would be skewed, had there been achange in elevation, and either b1 or b3 would rotate so as to move ontothe ER.) In View 3, beam 2's point of incidence has risen on the cornea,with the result that it is now reflected to c2.

Beams 1 and 3 may be kept on the ER by geometry changes in thephysician's prescription for the apparatus. Two examples of prescriptionchange to keep the reflections on the ER are offered here: first, theangles between beams as seen in the front view of View 1, which arenormal to the surface of the sclera as shown, may be increased (holdingthe central beam and points of incidence fixed) to give an increasingangle of incidence in this plane, hence, moving the a1 and a3 positionscloser to a2. Also, the pitch, or spacing between beam points ofincidence, may be reduced by reducing the angle between them, whilekeeping them normal to the sclera, to accomplish the effect of moving a1and a3 closer together. Regardless of details of the setup, the measuredresponse of the system at various IOPs affords the calibration.

Example 5

The prescription for the patient's system may be determined in thephysician's office by a system similar to those of FIGS. 10 or 15, butwith adjustability and verniers or other scales to show adjustmentdetails. Scans are made after the apparatus has been set to thephysician's satisfaction, to verify the function of the finalconfiguration of the adjustable system whose settings are used to definethat to be prescribed for the apparatus to be used in the field.

Calibration

The calibration is accomplished with the patient's personal apparatus ofeither separate or integral (to the eyeglass frame) type, and consistsof sweeps of the eye past the beam array at several values of IOP overas broad a range of IOPs as it is practical to induce at the time ofcalibration. Separate refers to a system that is packaged separatelyfrom the eyeglass frame which may be placed over the eye with glassesremoved for data acquisition. This allows for opaque seal to excludeambient light from interference with the proper function of theapparatus, but also eliminates automatic triggering.

The calibration procedure is never complete. Data from high pressure"events" in the physician's office will be recorded, verified byconventional tonometry, and entered into the calibration data file forcalibration extension at every possible occasion.

A curve, of reasonable range may be generated, in relatively short orderif a high pressure episode can be used to produce several levels of IOPby successive medication. Precise measurement of IOP during scan forrecord is made by a precision unit such as an applanation tonometer, andscans at several values of elevation may be made to establish thecorrelation between angularities on the ER, and the values ofdeflections of the beams that are proportional to IOP. This method isnot intended to replace precision measure of IOP; rather, it is a methodof detecting high pressure episodes, for correlation to the events thatcause them. The device is sufficiently quantitative that medication inproper dosage may be taken in time to prevent physiological damage. Thisinvention is sufficiently quantitative to document diurnal variations,resulting from patient routine, so important to treatment of glaucoma.Events may be cataloged and correlated with other patients with the sametype of glaucoma to aid treatment. It is difficult to overdose a patientwith pressure reducing drugs, therefore, significant reduction ofblindness will result from this self-monitor method of determiningoverpressure and proper medication.

IOP measurement by conventional means, with correlated sweeps are takenat each office visit as part of the patient's history; and thecalibration function is verified and expanded on a continuous basis.Changes in calibration are extremely important and give advance noticeof physiological change. Excessive IOP, in addition to causing damage tothe optic nerve, also can cause other permanent change in the eye. Thisis similar to the effects of engineering materials being stressed beyondtheir proportional limit, with resulting permanent distortion. Thisinvention offers the ability to detect and track such damage and tosuggest therapy and changes in patient routine to minimizedeterioration.

For final calibration the resulting peak deflections from limbuscrossing are tabulated against induced IOPs' and the deflection andangular signatures are analyzed and stored as calibration functions. Thevalues of induced IOPs are determined by conventional tonometry andentered into the patient's data unit and/or physician's computer,manually.

If the head is kept near vertical the rotation of the eye is restrictedsubstantially to rotations about the vertical axis. The elevation anglemodifies the angular deflections of the beams that are repeatablefunctions of contour, hence are measures of IOP. Recordings of thesesignatures at the different induced pressures and/or elevations producethe calibration reference.

Example 6

The acquisition of data with this unit should be automatic, if possible,particularly for the case where the diurnal variation of pressure isdesired. This is so that the pressures will be little effected by havingto think of, and manually prepare for, data capture. If preparation isrequired, the patient's response tends to be influenced by the act oftriggering the data. In general, the data are recorded directly indigital format as is consistent with the devices that are typical forthe ER. Several methods of triggering the recording of data may be usedas has been previously illustrated in FIG. 9.

Data Acquisition

Data from the ER may be monitored on a continuous basis so that when thetriggering event occurs, such as from the deflection of a beam toindicate IOP beyond a programmed limit, that data will be recorded inthe data unit. In addition to recording, the unit notifies the patientby sound or vibration that the data has exceeded program limits and thathe needs to take appropriate action. Triggering may be effected byseveral methods which include, but are not limited to:

A. Continuous monitor, of a "window in time" (fixed interval), withtriggering afforded by deflection of a beam on the ER in a mannersimilar to that for an oscilloscope. The "width" of the window allowsstorage of a precursive time increment (i.e., prior to triggering) toinsure capture of the entire event.

B. Manual actuation of a switch to "set" the trigger, followed by headturn to induce involuntary eye rotation; this is useful in recordingevents where the patient notices something that indicates he shouldrecord his IOP. Automatic disablement of this feature generally will beprogrammed to prevent interference with a previously triggered event,being recorded, that the patient is not aware of, though a "recording"light will be included on the data unit to indicate that recording is inprogress. This disablement also may be set to vary "window width," torestrict recording time and conserve memory.

C. Data may be triggered by combinations of programmed timing in thedata unit to arm the trigger to record the next limbus crossing assensed by the discontinuous step on the ER, or by notifying the patientby something, such as a vibrator that it is time to manually recorddata. Automatic triggering is helpful in establishing diurnal pressurevariations for the patient. Data may include date and time as part ofthe format. "Peak memory" update may be used to capture and storeextremes of pressure excursion triggered by routine eye motion thatsurpasses previously recorded deflection.

D. The simplicity of data format required to store IOP defininginformation (beam deflection and angle) makes feasible the continuousacquisition of data for 24 hours or more.

The beam sources should be as independent of ambient spectra as possibleto minimize background interference for the integral unit. This isinherent in the separate apparatus since background is eliminated. Whileit is desirable to maintain a cosmetically pleasing configuration forthe integral unit, it may be that a sealed or shaded "goggle" must beadopted to control ambient interference. It is possible to sampleambient spectrum as part of each data set and apply appropriatecorrection, but this complicates collection, reduction and correction ofthe data significantly. Each beam may be given a different character(such as color) for identification. The separate unit has no backgroundclutter, of course.

Example 7

For the integral system, the data unit may be kept "at the ready"continuously, or "armed" by a separate timing function, and triggered bysignal characteristics and conditions that may be either external to, orcontinuously monitored on, the ER itself. In this way the diurnalvariation of IOP during the patient's routine may be determined for histreatment in a manner that is impossible under monitor by conventionaltonometers.

Data Storage, Reduction and Processing

Once triggered, analog or digital data from the ER is recorded by thedata unit to store the paths of beam reflections as the beam crosses thelimbus. An example here is data from pixel by pixel illumination as abeam travels over a charge coupled device (CCD). The signature of alimbus arc segment may be constructed from the signatures of multiplebeams to characterize asymmetric corneal distortions in conjunction withthree dimensional mapping, drawing, or solid modeling software toidentify physical anomalies. Asymmetry may result if the cornea orsclera are stretched beyond their elastic limits. The single beam unitis expected to be sufficient for IOP measurement, in most instances.Notice that there is no real "zero" since there is always a finitelimbus angle. Analytical comparison techniques such as from Fourier orother geometric analysis may be used to extract secondary informationfrom the signatures that may be of value comparable to that of IOPitself, particularly with regard to similarity of response betweenpatients with the same type of glaucoma.

The data unit is as compact as possible, and provides dependable captureof IOP related contour signatures. The data unit contains an EPROM,programmed in accordance with prescription from the physician, tocontrol the storage of data from the ER in RAM. Data is recorded onstorage media such as magnetic tape or cards and may be communicatedfrom the patient to the physician's office for analysis. This allows thephysician to be involved in the diagnosis and treatment of the patient,to a degree and in a time frame, previously impossible for treatment ofglaucoma. The ability to know IOP in nearly real time will be veryeffective in preventing physiological damage that can cause blindness.FIG. 16 is a flowchart example of data acquisition, reduction, analysis,and handling for the field monitor of IOP or BP.

Since the eyelids are of relatively constant thickness, therebymodifying limbus contour in relatively constant fashion, the possibilityof making measurements with eyes closed is feasible, though reflectiveointment on the eyelids would be necessary to insure reasonablereflection. This concept is to be evaluated as development of thisinvention continues. The capability of measuring IOP through the eyelidwould be very effective in making automatic measurements of IOP duringsleep, when the body's supine position increases the systolic bloodpressure component of IOP due to the increase in the heart's heightrelative to the eye. In many instances the most damaging events to theeye, from glaucoma, occur during sleep.

Example 8

Similar apparatus and procedures may be used to measure blood pressure,blood chemistry, and pulse rate. The pupils of the eyes are the onlytransparent windows in the body where blood vessels and nerves may beobserved without an opaque barrier to their translucent walls. Bloodvessels are visible on the surface of the sclera, with a clarityunequaled elsewhere on the body. The opportunity to view these vesselsand nerves provides a unique opportunity to use signature analysistechniques to "learn" the nuances of physical shape and color inrelationship to health or disorder. The measurement of blood pressuremay be deduced from the distortions of blood vessels on the surface ofsclera or retina in a manner similar to that described for measuringIOP, i.e., through signature analysis of the reflections from thevessels, in direct comparison to calibration signatures recorded in thephysician's office.

Physiological condition related to the color of vessels and nerves arevisible in or on the eye. The color of these elements may be quantifiedby spectrum analysis, where the colors of specific elements may bediscretely analyzed to identify and quantify their presence. As anexample, the color of the optic nerve is directly related to its health;a healthy nerve being bright orange or pink; fading to a dull gray as itdeteriorates or dies.

Blood Pressure and optic Nerve

Similar apparatus to that described for the IOP may be used to measurecarotid artery distortions (i.e., changes in the artery's physicalsize/shape from blood pressure according to the equations of mechanics).The apparatus for making these measurements is not intended forcontinuous monitor, but to offer enhanced diagnostics of the patient'sgeneral health from scans of the eye during routine physician's officevisits. Positioning hardware to accurately locate the apparatus for suchapplications may be used to make complex, yet precise, physiologicalmeasurements in both clinical and non-clinical settings.

The images of the carotid arteries are irregular and are not suitablefor analysis by the simple beam deflection afforded by the limbusdiscontinuity for deducing IOP. The mapping of blood vessels and thereflections from them that are indicative of both systolic and diastolicblood pressure components requires that a two dimensional image beconstructed to identify specific vessels chosen for data and to recordthe complex reflections from them. This implies that a rasterized scanof the ER be employed (which then gives, in essence, a video camera).The output from this "camera" may be taken to a "correlator" to analyzethe similarity between the calibration recordings taken in thephysician's office, and the patient's field data, in the time domain(cross-correlation); the result being a measure of similarity betweenthe signals. Alternatively, the signals from the full ER matrix may becompared, digitally, to determine their similarities. Such similaritiesare qualitatively related to each other, and may be quantified bycomparison to the IOP measured separately on calibration. Further,images from the carotid artery may be compared with images of itselftaken at different times (auto-correlation) to quantify change inpatient condition. There are numerous other methods of comparing datasets; these are chosen for example, but are not the only means ofcomparison.

Determination of blood pressure also may be made from similar analysisof the surface blood vessels on the sclera; however, these vessels aresmaller, therefore less likely to give the resolution afforded from thecarotid. In order to get a good image of the carotid, through the pupil,it is necessary to get close enough to get a full view of the retinalplane. This problem may be alleviated by using a wavelength of lightthat is not in the visible range, allowing the pupil to remain inrelatively dilated condition.

Dynamically varying signals such as those from pulsations of bloodvessels due to diastolic variation may be analyzed by Fast FourierTransform (FFT) methods in a "spectrum analyzer" that processes signalsin the frequency domain to quantify harmonic content. In this instance,spectrum refers to the dynamic as opposed to color spectrum. Data fromthe ER, directly, or from the correlator above, may be edited into"endless loops," or repetitively played from digital storage, foranalysis in the frequency domain. Since correlation functions ofperiodic functions also are periodic, at the same frequency, correlationfunctions may be FFT processed in the frequency domain (cross powerspectral density) to sharpen the desired dynamic components such as fromdiastolic blood pressure variation. The diastolic components also may beisolated by digital comparison of images from successive raster scansfrom the ER, with the extreme variations defining the sum and differenceof systolic and diastolic components, respectively, all of which arerelated to the calibration images taken at known pressures.

The color spectrum of reflections from the nearly transparent walledcarotid artery may provide discretely identifiable signatures related toblood sugar, oxygen, alcohol levels, etc., to allow virtuallyinstantaneous blood chemistry analysis. The spectrum of the optic nerveis qualitatively related to its health, with a healthy nerve being abright red-orange or pink, fading to a dull gray as the nervedeteriorates or dies. Color spectrum signature analysis according tomodern methods such as from imaging spectrographs (Purcell, 1993)provides powerful diagnostics for general patient monitoring.Comparisons between spectra of patients with the similar disorders mayprovide direct diagnosis of numerous disorders.

In summary, the eye is the only transparent window in the body wheredirect observation of critical blood vessels and nerves, that respond tonumerous disorders, is afforded. Response to these disorders may becharacterized by observing signatures from the vessels or nervesthemselves and comparing them to related physiological functions thatare measured, independently, by conventional methods at the same time.Correlations between these phenomena allow recording of images innon-clinical settings from which deductions of the related functions areobtained in short time frame.

Example 9

The reduction of data for IOP is relatively simple and consists ofcomparing data defining the limbus contour against the calibrationstandards for that eye where data are taken under controlled clinicalconditions with separately measured pressure and angles of elevation.Data processing includes determination of the deflection and anglerelative to the ER and interpolation to give IOP.

Blood pressure, or BP, processing is more difficult and requires thatthe image of the vessel, chosen to determine the pressure relatedsignatures, be corrected for position and orientation before thecomparison is made. This implies that a reference point, and angularreference must be used to bring both calibration and data sets intophysical register with each other. Magnification may be reasonablystabilized by precision placement of the scanning unit (as may theangular orientation) which is connected to the patient's data unit ineither case. A separate unit seems superior to any continuously worn, or"integral" apparatus, since special provisions for precision placement,and exclusion of ambient interference would be better afforded.

Data Processing

As mentioned in the introduction above, determination of IOP is arelatively simple process involving the comparison of the peakdeflection of a data beam, as recorded in the patient's data unit to thebeam deflections that occurred with the same apparatus duringcalibration in the physician's office. This requires little beyondapplying linear interpolation between, or extrapolation beyond, thecalibration curve IOP increments, induced in the physician's office, tothe measured peak beam deflections observed in the data. The angularpath over the ER during beam deflection gives the elevation of the eye.The IOP is determined by double interpolation between angle and peakbeam deflection to give the IOP. Since the devices for an ER are of thedigital matrix type, or an analog type that may be converted to digitalformat, pixels defining rows and columns, sensitive to light give theinstantaneous position of the reflected beam. If data is recorded for awindow in time, the excursion of the beam over the ER is faithfullyrecorded. The "jump" at the discontinuity gives both limbus angle andazimuth related to the angle of elevation. Such data may be processedand compared with the calibration references in a fraction of a secondfor virtually real time indication of IOP.

Data reduction for BP measurement is more complicated. The image of thecarotid artery is complex, with little or no correlation betweenpatients; nor is there any symmetry or discrete geometric discontinuityas with the external surface of the eye at the limbus. The task is torelate the two-dimensional images, of the carotid or scleral vessels onthe ER, to BP. This requires that separate images of the carotid fromthe ER be compared, with the difference between them being the signatureto be related to pressure. In this instance, it is first necessary tocorrect for positional differences of images on the ER. Some majorfeature is chosen as "Zero" reference and the images are "corrected" tothe same reference.

It is expected that position and magnification may be sufficientlymaintained to preserve scale reference for the image. The referenceshould be as discrete a point as may be identified, for the specificpatient, and the alignment or rotational orientation of the image bepositively identified either by physical registry of the apparatus or bya second image comparison. There are numerous geometric analysisprograms that may be used to process the data from the pixels toidentify these features. This allows correction of the images to acommon origin, angular orientation, and scale. Differences betweensubsequent image matrices, for determination of systolic pressure anddiastolic variation, compared with the references taken for calibration,give the signatures that may be used to measure BP. Several comparisonsmust be made to establish both systolic and diastolic components ofblood pressure to insure that both high and low peaks of distortion fromdiastolic pressure are obtained. The determination of BP requires someprocess time, but may be quicker than that for conventionalsphygmomanometer and at a fraction of the size and weight. In thisinstance, the data unit will be larger than that for IOP due to thegreater requirement for full, multiple, image processing anddifferencing. Current "palm-top" computers have adequate storage andprocessing capability. The "separate" package for the sensors and ERarray may be of the size of a pack of cigarettes or so. These packagesizes may be expected to shrink with the trend in instrument developmentto smaller packages. Alternatively, the unit may be configured to storethe corrected images in a simple, and physically smaller, memory forsubsequent transmission to the physician's office for processing.Sequential images at high storage rates may be used to reconstruct thedynamic character of BP including "murmurs" or other anomalies.

Complex color spectral data may be recorded (Purcell, 1993). These dataare not expected to be highly dynamic; however, data will be evaluatedat sufficient data rates to determine if high rate changes in colorspectra occur during strenuous or stressful situations. The use ofspectrographic imaging may be used to isolate specific dynamictemperature variations from color photothermography (colors proportionalto temperature). While there might be some value in ambulatory monitorof color spectra, this is expected to be primarily a clinical unit usedfor checks during routine office visits, etc. A special CCD colorspectral processor could be made pocket size; again, with a separaterecording media for storing the images.

Reference

Purcell, Frank in Laser Focus World, "Imaging Spectrographs PerformedMultidimensional Spectroscopy," May 1993, pp. 93-97

What is claimed is:
 1. An intraocular pressure measuring apparatuscomprising:a light emitter means for emitting a light beam spaced orswept across the outer surface of a limbus region between the sclera andcornea of the eye; at least one photo detector spaced relative to theemitter wherein said photodetector converts to electrical signals theposition of a light beam reflected from the limbus region; ananalog-to-digital converter coupled to the photo detector that encodesthe electrical signals to digital data; and a local memory medium meansconnected to the converter for accumulating the digital data over aperiod of time wherein changes in contour of the limbus region that arecaused by alterations in intraocular pressure without contact or forceapplied to the eye measurably affect the electrical signals generated inresponse to the position of the reflected light beam.
 2. The apparatusas recited in claim 1, further comprising an eyeglass frame or lightopaque enclosure wherein said apparatus is mounted in fixed register tothe eye and adjustable positioned in close proximity to the limbusregion, the light emitter or multiple emitters and photo detector beingcoupled to a localized portion of said eyeglass frame or opaqueenclosure.
 3. The apparatus as recited in claim 2, wherein the lightemitter and photo detector are fixed in spaced relation to each otherand wherein the beam from the emitter angularly sweeps repetitivelyacross the limbus region.
 4. The apparatus as recited in claim 1,wherein the light emitter comprises a light emitting diode.
 5. Theapparatus as recited in claim 1, wherein the light emitter comprises alaser.
 6. The apparatus as recited in claim 1, wherein the local memorymedium comprises a read/write device carried with a patient undergoingintraocular pressure measurement or blood pressure measurement.
 7. Theapparatus as recited in claim 1 further comprising a remote memorymedium means for receiving downloaded digital data from the local memorymedium; anda signal processor coupled to the remote memory mediumwherein intraocular pressure and blood pressure are determined as afunction of the digital data.
 8. The apparatus as recited in claim 1,wherein the light emitter and detector are coupled to a scanner whichmoves the emitter and detector in close proximity across the outervessel surface.
 9. The apparatus as recited in claim 8, wherein thescanner comprises:a platform having the emitter and detector fixed inspaced relation to one another; and a motor drive source attached to theplatform for scanning the platform across the outer vessel surface. 10.The apparatus as recited in claim 1, further comprising a local memorymedium for storing the electrical signal comprising:A first remotememory medium means for accumulating a set of stored values to allowconversion of alterations in the electrical signal to changes inintraocular pressure; A second remote memory medium means foraccumulating a set of stored values to allow conversion of alterationsin the electrical signal to changes in blood pressure; and, A computermeans for converting connected to the first and second memory mediaalterations in the electrical signal to intraocular pressure and bloodpressure.
 11. A method for measuring changes in intraocular pressurewithin an eye, comprising:repeatedly scanning a light beam across alimbus region between the sclera and cornea of an eye without concurrentapplication of force or contact with the eye, producing a light beamreflected from the limbus region; repeatedly detecting alterations ofthe light beam reflected from the limbus region; and convertingalterations of the light beam to changes in intraocular pressure. 12.The method as recited in claim 11, wherein repeatedly scanning compriseslinearly moving the light beam across the limbus region.
 13. The methodas recited in claim 11, wherein repeatedly scanning comprisessubstantially continuously scanning the light beam across the limbusregion.
 14. The method as recited in claim 11, wherein alterations ofthe light beam reflected from the limbus region are measured as changesin intensity.
 15. The method as recited in claim 11, wherein alterationsof the light beam reflected from the limbus region are measured aschanges in position.